In a certain conventional gas turbine engine, an inner shaft supporting a low pressure compressor impeller wheel and a low pressure turbine wheel and an outer shaft supporting a high pressure compressor impeller wheel and a high pressure turbine wheel typically consist of hollow shafts which are coaxially nested with each other. Such a gas turbine engine is disclosed in WO98/28521 A1. According to the conventional gas turbine engine disclosed in this international patent publication, the outer and inner shafts are supported by separate bearings at their front and rear ends, and each bearing is lubricated by a forced lubrication system which blows lubricating oil fed by a pump to the bearing.
In such a forced lubricating system, to positively prevent the lubricating oil from leaking out of the bearing boxes provided in the front and rear ends of the outer and inner shafts, high pressure air drawn from the compressor and whose pressure and flow rate are controlled by using an orifice or the like is conducted to the exterior of the oil seals of the bearing boxes as seal air to keep the interior of the bearing boxes at a lower pressure than the exterior thereof. Also, a part of the high pressure air is used for cooling the turbine rotor.
The air (secondary air) for cooling and sealing can be supplied by bleeding an intermediate stage or final stage of the compressor (CDP) depending on the user of the secondary air and required pressure. Alternatively, the bleed air for sucking a boundary layer may be used for this purpose. In case of a centrifugal compressor, air may be bled by directing the flow issuing from the outlet of an impeller into a gap between the back side of the impeller and casing.
However, the gas turbine engines that supply air for cooling and sealing as described above have the following problems. (1) Because the high-pressure, high-temperature gas (secondary air) produced by the compressor is used for purposes other than providing a thrust, the overall efficiency of the engine is reduced. (2) When the high-temperature, high-pressure seal air is introduced into the bearing box, it causes the temperature of the lubricating oil to rise. This prevents adequate cooling of the lubricating oil, undesirably heats such components as the casing typically made of aluminum alloy and gears typically made of steel, and causes premature degradation of the lubricating oil.
To avoid such problems of the prior art, it was proposed in copending U.S. patent application Ser. No. 10/844,604 (publication No. US2005-0132706A1) to provide an impeller back chamber behind the centrifugal compressor and a high pressure air cooling turbine driven by the outer shaft in the impeller back chamber so that the flow of the high pressure swirl air conducted from the back side of the impeller into the impeller back chamber may be cooled by the high pressure air cooling turbine. According to this previous proposal, the high pressure swirl air that has flowed into the impeller back chamber flows into the turbine from the outer periphery thereof so that the energy is transferred from the high pressure air flow to the high pressure air cooling turbine, and the temperature of the high pressure air is lowered before it is supplied to the bearing boxes and turbine rotor as secondary air (for sealing and cooling purposes). Therefore, the energy loss of the engine is minimized, and the temperature of the lubricating oil in the bearing boxes can be controlled below a prescribed lever.
However, according to this previous proposal, the region radially inside the inlet (outer periphery) region of the high pressure air cooling turbine in the impeller back chamber forms a stagnation zone where there is little exchange of air with the outside. As the high pressure swirl air flows from the outer periphery (inlet end) of the impeller back chamber to the radially inner region, the swirl component progressively increases in proportion to the inward flow rate and is inversely proportional to the radius. However, the stagnation zone acts against the increase in the swirl component, and the friction of the air flow tends to obstruct the swirling of the high pressure swirl air even in the region outside the stagnation zone. This in turn prevents an adequate drop in the pressure of the high pressure swirl air, and causes an undesirable rise in the average pressure of the impeller back chamber.
If the average pressure of the impeller back chamber rises, because the front wall of the impeller back chamber is defined by the impeller wheel of the high pressure compressor, an axially forward load acts upon the impeller wheel (in other words, the outer shaft). As a result, the thrust acting upon the bearings supporting the outer shaft increases so that the resulting rise in the temperature of these bearings could lead to the premature degradation of the lubricating oil and adversely affects the durability of the bearings.